Electronic devices such as a personal computer include a switching power circuit (DC-DC converter) that supplies a drive voltage to an internal circuit that performs signal processing. The switching power circuit converts a direct current voltage supplied for example from an AC adapter or a battery to the drive voltage suitable for an operation of the internal circuit. The switching power circuit controls on/off of a main switch to generate an output voltage by raising and reducing the direct current input voltage, and in the meantime performs a feedback control to maintain the output voltage at a constant voltage corresponding to a load.
The main switch is for example a MOS transistor. The MOS transistor has a parasitic capacitance formed between a control terminal and other terminal (source terminal or drain terminal). A drive circuit drives the MOS transistor (turns the MOS transistor on and off) by charging or discharging a gate terminal of the MOS transistor (parasitic capacitance). A current flows from the gate terminal of the MOS transistor to a low potential power source (for example, ground) by the discharge, and this current becomes a loss to a power voltage. This loss is referred to as a Qg loss. Here, Qg is a charge amount that is accumulated in the gate terminal.
An amount of consumed current in a load of an electronic device changes depending on an operating state of the electronic device. A method to improve efficiency of the DC-DC converter upon a heavy load state requiring a relatively large amount of current is to make an on-resistance value of the MOS transistor small, that is, to make a size of the MOS transistor large. However, the charge amount Qg accumulated in the gate terminal is proportionate to the size of the MOS transistor. For example, making the size of the MOS transistor large increases the accumulated charge amount Qg, by which the loss is increased. For example, when the size of the MOS transistor is made small in order to reduce the loss, that is, to make the charge amount Qg small, the on-resistance value of the MOS transistor becomes high, and the conversion efficiency upon the heavy load state decreases.
Another method to reduce the loss is to supply the charges accumulated in the gate terminal to an output terminal of the DC-DC converter. The DC-DC converter for carrying out this method includes a switch coupled between the gate terminal of the MOS transistor and the output terminal, for example, and this switch is turned on or off upon switching the MOS transistor on or off. Due to this, the charges accumulated in the gate terminal are supplied to the output terminal. For example, refer to Japanese Laid-Open Patent Publication No. 2007-288935, US 2010/0308785, and US 2011/0001462.
As described above, in the MOS transistor including the gate terminal to which the switch is coupled, since an output terminal voltage is applied to the gate terminal by the switch being turned on, the on-resistance value becomes large compared to a case where the switch is not used. The increase in the on-resistance value leads to the efficiency reduction of the DC-DC converter.